Device for producing microstructures

ABSTRACT

The invention discloses a device and a method for producing microstructures with a spindle, which is adapted to be driven for rotation about its longitudinal axis and which is provided with a fixture for clamping a workpiece, having an actuator provided with a fast drive, in particular a piezo drive, adapted to produce fast movement of a tool in a direction substantially perpendicular to the workpiece surface, which actuator can be positioned along a workpiece surface to be worked with the aid of an additional drive adapted to produce a linear feed motion in a first direction, the fast drive being coupled with the tool via guide means that allow feeding of the tool in axial direction of the fast drive, against the action of a restoring force, and that is highly rigid in a plane perpendicular to that direction. The control technology used permits microstructures to be produced precisely and reproducibly utilizing the dynamic properties of the system.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of International Patent ApplicationPCT/EP2005/003860, filed on Apr. 13, 2005 designating the U.S., whichInternational Patent Application has been published in German languageand claims priority of German patent application 10 2005 020 990.1 filedon Apr. 23, 2004 and of German utility model application 20 2004 011815.7, filed on Jul. 28, 2004. The entire contents of these priorityapplications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a device for producing microstructureswith a spindle, which is adapted to be driven for rotation about itslongitudinal axis and which is provided with a fixture for clamping aworkpiece, having an actuator provided with a fast drive adapted toproduce fast movement of a tool in a direction substantiallyperpendicular to a workpiece surface, which actuator can be positionedalong a workpiece surface to be worked with the aid of an additionaldrive adapted to produce a linear feed motion in a first direction.

The invention further relates to an actuator suited for a device of thatkind.

Finally, the invention relates to a method for producing amicrostructured surface on a workpiece which can be rotated by a spindleusing a tool that can be driven by an actuator and can be positioned inlinear direction along a workpiece by means of a drive and can be movedby the actuator toward the workpiece surface in perpendicular directionrelative to that linear direction.

A device and a method of the described kind are known from EP 0 439 425B1.

The known method and the known device are used for producing contactlenses by a turning process. According to that process, a blank for acontact lens to be worked is mounted on a spindle. The workpiece, beingrotationally driven by the spindle, can be worked by a turning toolwhich is positioned with the aid of a piezo drive. The turning machinecomprises a column-like swivel head seated for pivotal movement about apivot axes. A carriage guide is mounted on the swivel head. The carriageguide serves to guide a tool carriage in linear and radial directionrelative to the pivot axis of the swivel head. A tool holder for theturning tool, preferably in the form of a turning diamond is supportedon the tool carriage. For fast feed of the turning tool about the pivotaxis of the swivel head, the tool carriage can be moved along thecarriage guide by means of a motor drive. For fine positioning of theturning tool, a piezo drive is provided which may consist of two piezotranslation means. While a first piezo translation means allows amovement to be performed in the direction of the carriage guide, asecond piezo translation means is adapted to permit movementperpendicular to that first movement. The positioning movements of thepiezo translation means may be controlled in response to the angle ofrotation of the spindle about the spindle axis.

This arrangement permits highly precise surface working of lenses by adiamond cutter, and permits even rotationally non-symmetrical surfacesto be produced.

Similar devices for producing rotationally non-symmetrical surfaces byturning with the aid of a piezo drive have been known, for example, fromU.S. Pat. No. 5,467,675 or GB 2 314 452 A.

Although the known devices and the known methods allow fine-structuresurface working using diamond tools, such devices and methods, due toinadequate dynamic properties, are not suited for working very hardmetal materials with high precision. Such materials are required, forexample, as mold materials for the production of lenses by hot-pressing.The materials in question may, for example, be hard alloys. In theproduction of lenses for illumination purposes, which are used inspotlights known as poly-ellipsoid spotlights (PES spotlights) it is,for example, necessary to provide such molds with very specificmicrostructures which are then transferred to the respective lensesduring the hot-pressing process. The microstructures serve asmicro-optical components in the lenses so produced in order to meetgiven predefined light intensity distribution characteristics for thespotlight.

The production of such molds, consisting of a hard alloy or of castiron, for example, has been possible to this day in the envisaged formonly with the aid of geometrically uncertain processes. The molds areinitially worked in the macroscopic form of the respective lenses byturning, where after the mold surface is polished, if necessary.Thereafter, the areas of the mold from which scattering centers are tobe created on the lenses to be produced are produced by a number ofoperations, for example by a corundum blasting process in which masksare employed to cover those areas on which no microstructures are to beproduced in that way. The blasting process is then followed, in part, bya two-dimensional after-treatment. Thus, for producing such a mold,numerous manual operations are required, which means that extremelytime-consuming and expensive working is necessary to produce the desiredsurface structure. In addition, such a sequence of operations is highlysusceptible to errors and faults which has an adverse influence on thereproducibility of the given light intensity distributioncharacteristics.

The before-mentioned known devices with piezo drives are not suited forsuch shaping operations, lacking the mechanical stability required forthat purpose and the dynamic properties necessary for working hard metalpiezo workpieces precisely at the required cutting speeds.

While position-controlled working may be possible with known devices,this naturally can be done only with a sufficiently big distance fromthe first resonant frequency of the respective system, which means thatworking with the known systems is possible only up to approximately1,000 Hz maximally. The cutting speeds so achievable are, however,insufficient for working the before-mentioned materials properly, withadequate surface quality, in particular when rotationallynon-symmetrical surface structures of the before-mentioned kind are tobe produced.

SUMMARY OF THE INVENTION

It is a first object of the present invention to provide a device and amethod for producing microstructures which permits even hard metallicmaterials, such as hard alloys and cast iron, to be worked with highcutting speeds and which simultaneously allows rotationallynon-symmetrical microstructures to be achieved.

It is a second object of the invention to provide a device and a methodfor producing microstructures on molds for the production, byhot-pressing, of lenses which are suited as lenses for PES spotlightsand are provided with scattering lens microstructures.

It is a third object of the invention to provide a device and a methodfor producing microstructures by a turning operation with high cuttingspeed on workpieces consisting of a very hard or brittle material, suchas hard metal or cast iron.

These and other objects of the invention are achieved with the aid of adevice of the type described above in that the actuator is coupled withthe tool via guide means that allows feeding of the tool in axialdirection of the fast drive, against the action of a restoring force,and which is highly rigid in a plane perpendicular to that direction.

By coupling a fast drive with custom-designed guide means, having highrigidity in all directions in a plane perpendicular to the direction ofmovement of the fast drive, it is possible to provide anactuator-controlled system for movement of a turning tool that exhibitsa sufficiently high resonant frequency in combination with high dynamicrigidity to allow even hard materials, such as hard alloys or cast iron,to be worked with sufficiently high cutting speeds. At the same timevibrations, which otherwise would occur when working hard materials,such as cast iron, and which would impair the surface quality, areavoided in this case.

Preferably, the fast drive is a piezo drive.

However, other embodiments of the fast drive, such as a hydraulic drive,are likewise imaginable. The term fast drive as used in connection withthe present invention describes a drive which is capable of performing afast controlled movement in the axial direction, but is not capable ofperforming a controlled movement in a plane perpendicular to thatdirection and can take up very small forces only in that latterdirection. The term “fast” means in this connection that the drive, whendriven by a specific control signal, can perform a movement at afrequency of at least 500 Hz or over.

According to a preferred further development of the invention, the guidemeans has a static rigidity of at least 50 N/μmm preferably of at least100 N/μm, in a plane perpendicular to the feed direction of the guidemeans.

Such a device allows in particular the production of molds forhot-pressing of lenses for PES spotlights, the surfaces of which areprovided with microstructures or are “frosted”. Such a device permitscutting speeds in the order of 60 meters per minute to be achieved whenworking cast iron or hard alloys. This is sufficient to guarantee properworking of the rotationally non-symmetrical surfaces in an acceptableperiod of time. The inserts used in this case may be of a conventionaltype.

According to an advantageous further development of the invention, theguide means comprises a ram preferably supported on first and secondspring elements, the spring elements being largely unyielding in theradial direction of the ram, while allowing deflection in the directionof the ram axis against the spring force of the spring elements.

According to an advantageous further development of that embodiment, thespring elements are configured for this purpose as leaf springs thathave their lengthwise ends clamped in holders and can be movedtransversely to that direction.

According to a convenient further development of that embodiment, thespring elements in that case consist of feeler gauge strips that areclamped in crossed arrangement between the holder and the ram in theradial direction.

Alternatively, a configuration as radially symmetrical spring elements,for example in the form of disk springs, is likewise possible, in whichcase the ram would be coupled at the center.

Such a structure allows sufficient resilience of the guide means in thedirection of movement of the fast drive to be achieved, whilesimultaneously providing high rigidity in a plane perpendicular to thatdirection.

According to an advantageous further development of the invention, thetool is clamped at a first end of the ram while the ram is biasedagainst the fast drive at its second end opposite the tool.

This guarantees the required restoring force against which the fastdrive acts.

According to an advantageous further development of that embodiment, theram is biased against the fast drive by fluid pressure.

Although biasing may generally be achieved also by mechanical means,such as by springs, this feature has the effect of reducing the effectsof cutting-power noise.

According to an advantageous further development of that embodiment, theram is held in a housing, the housing and the ram forming apressure-tight space to which a fluid pressure can be applied, whichspace is sealed toward the outside in axial direction by a firstdiaphragm connected with the ram on the piezo side, and by a seconddiaphragm connected with the ram on the tool side, and the firstdiaphragm having a larger active surface exposed to the fluid pressurethan the second diaphragm.

According to an advantageous further development of that embodiment, thediaphragms consist of aluminum.

It is possible in this way to guarantee the restoring force for thepiezo drive in a simple and reliable fashion and, at the same time, toreduce the possible effects of cutting-power noise.

According to an advantageous further development of that embodiment, thehousing comprises at least one damping element made from a sinteredmaterial, preferably a sintered metal. The damping element preferablyexhibits in this case an amount of open porosity.

In addition in this design a gap having preferably a thickness between0.1 and 1 millimeters and being filled with a damping medium may beformed between the damping element and the associated membrane. Thedamping medium may e.g. be air, grease or oil.

These features help to further reduce the effects of cutting-powernoise.

According to an advantageous further development of the invention, thefirst end, i.e. the end opposite the ram, of the piezo drive is clampedon a holding fixture while its second end, opposite the first end, iscoupled with the ram via a compensating element which is capable ofcompensating alignment errors between the ram axis and the longitudinalaxis of the piezo drive.

In this case, the second end of the piezo drive may rest against anassociated centering means of the ram via a convex element, for example,in particular a ball or a spherical element.

It is possible in this way to largely eliminate the application ofstatic and dynamic transverse forces in radial direction, which mightoccur, for example, by alignment errors between the piezo drive and theram and as a result of cutting-power noise.

According to an alternative embodiment of the invention, the second endof the piezo drive ends in a cover plate which is coupled with the ramvia a necked portion.

In this way, the mass of the entire system can be still further reducedand, if necessary, one joint can be saved, which is of advantage interms of the dynamics of the overall system.

According to a further development of the invention, the drive permits afeed motion in a direction perpendicular to the spindle axis, while theactuator permits a movement of the tool in the direction of the spindleaxis.

With this embodiment, the turning operation is effected in the way of afacing operation, as the tool is being fed by the piezo drivesubstantially in the direction of the spindle axis, i.e. in z direction.

Such a structure is suited, for example, for producing microstructureson molds for the production of lenses for PES spotlights.

According to a further embodiment of the invention, the drive performs afeed motion in a direction parallel to the spindle axis, while theactuator permits the tool to be moved in a direction perpendicular inthat direction.

In the case of that arrangement, surface working of the workpiece, byproducing microstructures on its outer surface, is possible inlongitudinal direction. Accordingly, the arrangement works in the way ofa longitudinal turning machine, with the actuator allowing the tool tobe fed in a plane perpendicular to the spindle axis, i.e. for example inx direction.

Such an arrangement is suited, for example, for the production ofmicrostructures on surfaces subjected to tribological stresses, such asouter bearing surfaces, for example for the purpose of improvingadhesion of grease, to thereby achieve clearly improved lubricationproperties and anti-seizure performance.

According to a convenient further development of the invention, there isprovided an electronic control for controlling the movement of theactuator, which controls the movement of the actuator relative to theworkpiece in response to the angular position of the workpiece and theposition of the actuator along the first direction.

This allows rotationally non-symmetrical surface structures to beachieved with the aid of the piezo feed motion of the tool.

According to a further configuration of the invention, the controlcomprises means for transforming a specified microstructure to beproduced in the mold, defined by Cartesian coordinates, to acoordinate-transformed structure defined by polar coordinates, where theactuation values are stored as a function of polar coordinatescontaining the angle of rotation and the radius.

Such coordinate transformation allows the respective actuation valuesfor the actuator to be determined in the case of facing operations.

The coordinate-transformed structure is stored in this case preferablyin a look-up table (LUT), from which the electronic control derives anactuation value that is supplied to an amplifier for the purpose ofcontrolling the actuator.

According to a preferred further development of that embodiment, theelectronic control comprises means for interpolation of the actuationsignal supplied to the amplifier as a function of position increments ofthe linear feed motion in the first direction.

While interpolation is rendered superfluous by the inertia of themechanical system in the working direction of the piezo drive, whichautomatically has a certain smoothing effect, interpolation of theactuation signals supplied to the amplifier as a function of positionincrements of the linear feed motion makes sense. One thereby avoids theformation of marks similar to tool marks produced by the linear feedmotion in radial direction as adjacent pixels of the microstructure arebeing mapped.

According to a further advantageous embodiment of the invention, theactuator has a first resonant frequency of at least 1,500 Hz, preferablyof at least 2,000 Hz, more preferably of at least 3,000 Hz, the controlmeans being designed to supply the actuator with a low-pass filtered orband-pass filtered signal whose upper cut-off frequency is below theresonant frequency of the actuator.

This embodiment makes it possible, without position control of theactuator, to work with a high cutting speed which is selected to ensurethat the cut-off frequency of the desired microstructure to be producedis slightly below the resonant frequency of the actuator. It is thuspossible for an open-loop system without position control to work withthe highest possible cutting speed that is still sufficiently spacedfrom the resonant frequency of the actuator.

The object of the invention is further achieved by an actuator adaptedto move the tool of a turning machine for producing a microstructuredsurface using a piezo drive, the piezo drive being coupled with the toolvia guide means that allow the tool to be fed in the axial direction ofthe piezo drive, against the action of a restoring force, and thatexhibits high rigidity in a plane perpendicular to that direction.

With respect to the method, the object of the invention is achieved by amethod for producing a microstructured surface on a workpiece rotated bya spindle, using an actuator-driven tool that can be moved in thedirection toward the workpiece surface by an actuator and can belinearly positioned, in a direction perpendicular thereto, along theworkpiece surface by a further drive, the method comprising thefollowing steps:

-   (a) Providing a desired microstructure for a workpiece to be worked;-   (b) transforming the desired microstructure to a file (look-up    table, LUT) containing actuating positions in a perpendicular    working direction as a function of the angle of rotation of the    workpiece and the linear feed travel of the tool along the workpiece    surface, for the piezo-controlled feed motion of the tool;-   (c) performing a spatial frequency analysis of the desired    microstructure, and determining a maximum cut-off frequency of the    signal for the feed position as a function of the angle of rotation,    the linear feed motion of the actuator along the workpiece surface    and of the cutting speed;-   (d) setting the cutting speed for the turning operation of the    workpiece so that the maximum cut-off frequency of the desired    microstructure is lower than the first resonant frequency of the    actuator;-   (e) driving the spindle and a drive for linear positioning of the    actuator along the workpiece surface and producing microstructures    on the workpiece by feeding the tool against the workpiece surface    by means of the actuator based on actuation values derived from the    look-up table as a function of the cutting speed, the angle of    rotation and the length of the feed motion of the actuator along the    workpiece surface.

The method according to the invention thus ensures that inmicrostructuring the workpiece, the highest possible cutting speed canbe used while still operating the actuator at a frequency below itsresonant frequency. This allows the system to be utilized up to itsmaximum cutting speed defined by the resonant frequency of the actuator,without a position control being necessary.

The method according to the invention basically is also suited for usein combination with conventional devices for microstructuring workpiecesby a turning operation using actuator-controlled tools. Preferably,however, the method according to the invention is used in combinationwith a device according to the invention.

According to an advantageous further development of the method accordingto the invention, the signal for the feed motion of the actuator islow-pass filtered or band-pass filtered, if the cutting speed that canbe adjusted according to step (d) is insufficient.

In the event spatial frequency analysis of the desired microstructure tobe produced shows that very high frequencies of the kind occurring, forexample, in the case of sharp edge transitions, are contained, thecutting speed to be selected normally would be clearly reduced tomaintain sufficient spacing from the resonant frequency of the system.The feature described above ensures in this case that a high cuttingspeed can be used even in the case of such a desired microstructure. Byinitially filtering the signal, the desired microstructure is sort ofsmoothed to ensure that a sufficiently high cutting speed can be usedfor working, for example, hard materials such as cast iron or hardalloys.

According to a further embodiment of the invention, the desiredmicrostructure of the workpiece is produced with the aid of an algorithmin such a way that low-pass limited white noise occurs in the spatialfrequency analysis.

The desired microstructure of the workpiece can be produced in this caseby a dot pattern generated by a random-check generator, folded by adeep-pass filter, preferably a binomial filter.

With this configuration of the microstructure, it is possible to ensurealready during production of the desired microstructure that a low-passfiltered signal is obtained which is in particular well suited forworking slightly below the resonant frequency of the actuator.

Alternatively, such a desired microstructure can be produced bygenerating a randomly generated dot pattern which is then transformed toa frequency space, is low-pass filtered and retransformed to the localspace.

Such a desired microstructure is in particular well suited for theproduction of a mold for fabricating a lens for a PES spotlight byhot-pressing, where a scattering lens microstructure is to be formed onthe lens surface.

According to the method of the invention, the actuator advantageouslycan be fed in the direction of the spindle axis and can be positioned bythe first drive in a direction perpendicular to that direction.

This permits working in the way of a facing operation.

Alternatively, the actuator can be positioned by the first drive in thez direction, parallel to the spindle axis, and can be fed against theworkpiece surface in a direction perpendicular to that direction.

In the case of that embodiment, microstructuring of the workpiece isrendered possible in the way of a longitudinal turning operation.

As has been mentioned before, the method according to the invention isin particular well suited for microstructuring an optical element, inparticular a lens microstructure or a scattering structure, or for theproduction of hot-pressing mold for such an optical element.

In addition, the method according to the invention is suited fornumerous other purposes in which microstructures are to be produced onworkpiece surfaces. This includes, among other things, the production ofmicrostructures on a surface subjected to tribological stresses, inparticular for a friction bearing.

It is understood that the features of the invention mentioned above andthose yet to be explained below can be used not only in the respectivecombinations indicated, but also in other combinations or in isolation,without leaving the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will be apparent fromthe description that follows of certain preferred embodiments, withreference to the drawing in which:

FIG. 1 shows a greatly simplified diagrammatic representation of adevice according to the invention;

FIG. 2 shows a longitudinal section through an actuator according to theinvention as shown in FIG. 1;

FIG. 2 a shows a detail of the actuator according to the invention,slightly modified compared with FIG. 2, illustrating the area of theconnection between actuator and ram;

FIG. 3 shows a perspective view of the actuator according to FIG. 2;

FIG. 4 shows the frequency response characteristic and the phaseresponse characteristic of the actuator with tool, according to FIG. 2;

FIG. 5 shows an enlarged detail of a desired lens microstructure to beproduced for fabrication of the mold for the production of a lens for aPES spotlight;

FIG. 6 shows a representation of the desired microstructure according toFIG. 5 as a pixel map;

FIG. 7 shows a spatial frequency analysis of the desired microstructureaccording to FIG. 6, along the outer periphery at a radius of R=34 mm;

FIG. 8 shows a block diagram including the control for the actuator;

FIG. 9 shows a diagrammatic representation of the algorithm for drivingthe acuator; and

FIG. 10 shows a longitudinal section through an actuator according tothe invention which is slightly modified with respect to the embodimentshown in FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, a device according to the invention is shown verydiagrammatically and is indicated generally by reference numeral 10.

The device 10 according to the invention is a turning machine which isadditionally equipped with an actuator 30 in order to permit fast, piezocontrolled movement of a tool 32 relative to a workpiece 16. The device10 comprises a spindle 12 that can be driven for rotation about itsspindle axis 25, as indicated by arrow 28. Mounted on the spindle 12 isa fixture 14 in the form of a chuck adapted to clamp a workpiece 16.

A drive 20 is provided on a machine bed 13 of the device 10, along aguide extending in the z direction (parallel to the spindle axis 25), bymeans of which a tool carriage 18 can be displaced in z direction, asindicated by double arrow 26. The tool carriage 18 is provided with adrive 22 by means of which a carriage 24 can be displaced in verticaldirection (y direction) as indicated by double arrow 27. The carriage 24in turn is provided with a further drive allowing it to be moved inhorizontal direction (x direction, perpendicular to the spindle axis25).

The carriage 24 finally supports the actuator 30 by means of which thetool 32 can be additionally displaced in z direction, as indicated bydouble arrow 34.

For controlling the turning machine and the actuator, there is provideda central control, indicated diagrammatically by reference numeral 17.The angular position of the spindle 12 can be monitored by an encoder31.

Now, for producing a microstructure on the surface of a workpiece 16,the device 10 can be used in such a way that the workpiece 16 is rotatedby the spindle 12 about the spindle axis 25 while the actuator 30 ispositioned in the x direction by the carriage 24, and the tool 32 is fedin x direction toward the workpiece surface by the actuator 30 inresponse to the angular position of the workpiece 16, determined by theencoder 31, and the spatial coordinate of the actuator 30. In this way,microstructures can be produced on the workpiece surface, androtationally non-symmetrical surface structures can be produced withhigh precision.

The structure described with reference to FIG. 1 allows workpieces to beworked in the way of a facing operation, which means that the workpieceis set into rotation and the tool 32 is moved predominantly inhorizontal direction (x direction), transversely to the spindle axis, orin radial direction to the workpiece 16, while the fast feed motion ofthe actuator 30 occurs in z direction, i.e. in the direction of thespindle axis 25. Other movements in other directions, for example in zdirection for the purpose of turning contours, are of courseadditionally possible.

It is understood that working the workpiece in the way of a longitudinalturning operation is likewise possible. In this case, the positioningmovement of the actuator 30 is effected by the drive 20 in z directionwhile a feed motion is rendered possible by the actuator 30 in a planeperpendicular to that direction (x/y plane). Conveniently, the actuatoris positioned for this purpose on the carriage 24 in such a way that thepiezo axis extends in x direction (or else in y direction). Such anarrangement is suited, for example, for producing a microstructure onthe outer surface of a workpiece (or on its inner surface) in the way ofan external cylindrical turning operation or an internal cylindricalturning operation.

The particular structure of the actuator, which can be moved very fastlyin the direction of the piezo axis, against the action of a restoringforce, but which exhibits high rigidity in a plane perpendicular to thepiezo axis, will be described hereafter in more detail with reference toFIGS. 2 and 3.

The actuator illustrated in FIGS. 2 and 3 and indicated generally byreference numeral 30 consists essentially of a piezo drive 36, receivedon a housing and acting on a tool 32 via guide means 38. The piezo drive36 allows movement only in the axial direction of the piezo drive, andis incapable of producing feed power in a plane perpendicular to thatdirection. Now, it is guaranteed by the guide means 38 that feedmovements of the piezo drive 36 in its axial direction can betransmitted directly to the tool 32, while the entire actuator 30exhibits high rigidity in a plane perpendicular to the piezo axis.

The guide means 28 comprises for this purpose a ram 40 whose ram axis 41is aligned as exactly as possible to the longitudinal axis 37 of thepiezo drive 36. The ram 40 can be moved along its ram axis 41 in axialdirection, against the force of first spring elements 44 and secondspring elements 48, but is suspended in practically unyielding fashionin a plane perpendicular to the ram axis 41, due to the special designand clamping arrangement of the spring elements 44, 48. The springelements 44 and 48 are leaf springs made from spring steel of greatwidth and small thickness, as known for example from the use as feelergauge strips. The outer ends of the leaf springs are clamped in holderswhile the ram 40 is mounted in the middle of them. The first springelements 44 are clamped between annular holders 45, 46, while the secondspring elements 48 are clamped between annular holders 49, 50.

All in all, one obtains in this way a possible axial excursion of theram in the order of approximately 0.5 mm, whereas the ram 40 is heldvery rigidly in a plane perpendicular to that direction (static rigidityabove 100 N/μm). The necessary restoring force acting on the piezo drive36 is produced pneumatically, not mechanically, in this case. The ram 40is suspended for this purpose in a housing 64 in such a way that acavity 72 is achieved, which is sealed air-tight toward the outside andwhich can be supplied with compressed air via a compressed-airconnection 74. The cavity 72 is sealed in the axial direction, at itsend facing the piezo drive 36, by a first diaphragm 68 and at its endfacing the tool by a second diaphragm 70. The diameter effective to theoutside (to ambient air) of the first diaphragm 68, that faces the piezodrive 36, is clearly greater in this case than the diaphragm diameter 70on the opposite side. This results in a differential pressure whichbiases the ram 40, which is connected by the two diaphragms 68, 70 atthe center, in the direction of the piezo drive 36. The biasing force,which of course should be greater than the acceleration forces producedby the piezo drive 36, can be adjusted by proper selection of thesurface area relationships of the diaphragms 68, 70 and by the pressureapplied.

The diaphragms 68, 70 preferably consist of relatively rigid aluminumelements and are each in contact, by their outer peripheries, with athin counter-diaphragm 69 or 71, respectively, which are pressed againstthe diaphragm 68 or 70, respectively, due to the overpressure prevailingin the cavity 72. Further, a sintered metal plate 66, which largelyextends over the entire cross-section of the cavity 72, is retained at avery small distance from the counter-diaphragm 69 on the axial end ofthe cavity 72 facing the piezo drive. The sintered metal plate 66consists, for example, of sintered steel and exhibits a certain amountof open porosity.

The sintered metal plate 36 is, therefore, permeable to air so that thepressure prevailing inside the cavity 72 can be transmitted to thediaphragm 68, whereas the thin channels formed by the open porosity leadto a notable damping effect whereby the effects of cutting-power noisein operation of the actuator are clearly reduced.

The tool 32 as such may be configured, for example, as insert mounted ona fixture 42, which latter is mounted on the outer axial end of the ram40 (by means of a screw not shown), the mounting on the second springelements 48 being simultaneously realized by the crossed spring strips.The opposite end of the ram 40 is firmly connected with the two firstcrossed spring elements 44. This is effected by a screw 80 which extendsthrough corresponding central recesses in the two crossed springelements 44 and an intermediate spacer 76 as well as an intermediatepiece 78, and into the end of the ram 40 with which it is connected byscrewing.

The piezo drive 36 has its end, that faces away from the ram 40, rigidlyjoined by a screw connection with an end piece 60, via a piezo seat. Atits end facing the ram 40, the piezo drive 36 is in contact with thescrew 80 via a ball 82 and a centering element 86. One avoids in thisway any radial forces acting on the piezo drive 36, that may result fromalignment errors between the longitudinal axis 37 of the piezo drive 36and the ram axis 41. It is thus ensured that the piezo drive 36 will beloaded in the direction of its longitudinal axis 37 only.

The unit formed by the piezo drive 36 and its suspension and the endpiece 60 is screwed together directly through the holders 45, 46 viathreaded bolts 52 and intermediate spacer sleeves 54.

The entire arrangement thus provides a compact actuator 30 where thepiezo drive 36 can be moved in axial direction against a pneumaticrestoring force and exhibits high rigidity in a plane perpendicular tothat direction.

Additional features, such as complete splash protection suited toprotect the piezo drive 36 from cutting oil during the turningoperation, need not be described here in detail, being commonly known toany man of the art.

A modification of the connection between the piezo drive 36 and the ram40 can be seen in FIG. 2 a. Here again, a compensation element 43 isprovided between the piezo drive 36 and the ram 40, for compensatingpotential alignment errors between the longitudinal axis 37 of the piezodrive 36 and the ram axis 41. However, the compensation element 36 isconfigured in this case as necked portion 47 coupled directly with anend plate 39 of the piezo drive 36 and with the ram 40, therebyrealizing a direct, but laterally flexible connection between the piezodrive 36 and the ram 40.

There can be additionally seen in FIG. 3 a lateral bracket 84 by meansof which the actuator 30 can be mounted on the carriage 24, for example.

FIG. 4 now shows the measured frequency response characteristic and theworking range of the actuator according to FIGS. 2 and 3. In the presentcase, a piezo crystal with a maximum feed of 40 μm was used as piezodrive 36. The piezo drive 36 was excited with an amplitude of 1 V in afrequency range from 0 to 4,000 Hz. The upper diagram in FIG. 4 showsthe travel as a function of the excitation voltage. The vibrometertravel recorded directly on the tool 32 shows a very constant amplitudecharacteristic of approximately 2.5 μm/V of 0 to approximately 2250 Hz.The first resonant frequency is approximately 2,400 Hz.

The lower portion of the diagram of FIG. 4 shows a representation of thephase response characteristic as a function of the frequency. The phaseresponse characteristic is very linear, from 0 to 2250 Hz.

Due to its very good dynamic properties, the actuator according to theinvention is, thus, in particular well suited for producingmicrostructures on surfaces, in particular for machining operations onhard materials, for which high cutting speeds are required. In the caseof a structure amplitude of 5 μm, for example, feed frequencies of up to2.25 kHz can be realized. For higher feed amplitudes, or very greatmaterial removal rates, the maximum feed frequency would by somewhatlower. A further improvement of the dynamics by the use of aconventional closed-loop control (instead of the open-loop control usedhere) does not seem to be necessary, and would not be possible up toslightly below the resonant frequency, due to the measured phaseresponse characteristic.

The maximum actuator travel of the actuator 30 used is approximately 40μm, so that the resonant frequency is in the range of approximately2,400 Hz. In many applications, however, a piezo drive with a maximumactuator travel of 20 μm would be sufficient. This would then result inan increase of the lowest resonant frequency to approximately 3.5 kHz.

The selection of the correct working conditions for the production of alens microstructure will now be described in more detail with referenceto FIGS. 5 to 7.

The actuator 30, which has been described in detail above with referenceto FIGS. 2 to 4, was used in connection with a turning machine of thetype Index built in 1978, which comprised a highly precise spindle witha radial eccentricity in the order of 0.4 μm.

The device was used for the production of a mold for hot-pressing oflenses for PES spotlights. In order to comply with given light intensitydistribution characteristics, selected surface areas of the respectivelens must be provided in this case with a microstructure in the form ofmicro lenses. Such a microstructured or “frosted” lens leads to speciallight intensity distribution characteristics when used in the respectivepoly ellipsoid spotlight. The microstructure of the surface defined inthe mold is transferred to the lens surface during production of thelens by hot-pressing.

FIG. 5 now shows an enlarged detail of such a microstructure. The greyvalue plotted on the vertical axis is h_(min)=0 and h_(max)=255, whichcorresponds to a peak-to-valley depth of 0 to approximately 10 μm.

Such a structure can be produced also by a randomly generated dotpattern, folded by a binomial filter. For producing such a structure, asillustrated in FIG. 6, individual pixels may be addressed by arandom-check generator, for example in an image 801 b 801 pixels big. Ifno pixel has been addressed before in a spatial distance of n pixels(i.e. all grey values of neighboring pixels in that circumscribed circleare 0), then the grey value at the addressed pixel is set to 1. Therandom-check generator performs several hundred thousand cycles in thisway. The result is an image with randomly ordered pixels of the greyvalue 1, where every pair of neighboring pixels does not fall below thatlimit. The image is then folded two-dimensionally by the binomialfilter, if necessary a plurality of times. Speaking figuratively, thebinomial filter is “put on” each of the individual pixels of grey value1 at the center. The result is an image with the randomly arranged“peaks” and “valleys” which very efficiently simulates the shape of alens microstructure according to FIG. 5. As a binomial filter of thatkind has low-pass properties, the entire image has low-pass propertiesas well. This fits in very well with fabrication using the actuatoremployed.

In the illustrated case the microstructure illustrated in FIG. 5 or FIG.6, respectively, is to be applied on the surface of the mold by a facingoperating using the actuator 30. It is of course necessary in this caseto convert the desired microstructure, defined by Cartesian coordinates,to a polar system of coordinates. This structure, converted to polarcoordinates, may be stored in the form of a look-up table (LUT), forexample. This table then defines the output value G(c,n) for theactuator as a function of the angle of rotation (c) and of the position(n) of the actuator in radial direction.

FIG. 7 now shows a spatial frequency analysis derived from a structureaccording to FIG. 6. The spatial frequency is recorded in this case onthe outer circumference of the structure to be investigated (in thepresent case at a radius of 34 mm) by FFT Fourier analysis (fast Fouriertransform). From the illustrated surface spectrum over a circumferentialsection it appears that the spatial frequency response characteristic isrelatively constant from zero to approximately two periods permillimeter. Starting at approximately 2.8 structures per millimeter, thespatial frequency response characteristic is near zero.

The lower representation in FIG. 7 illustrates the conversion of thespatial frequency characteristic recorded by means of FFT analysis to anoscillation frequency characteristic, based on an assumed tool cuttingspeed of 50 m/min. The result is a linear drop of the spatial frequencycharacteristic to near zero after 2,350 Hz, while a very linear phasecharacteristic exists up to approximately 1.7 kHz.

According to the invention, the cutting speed is selected to ensure thatthe amplitude response characteristic will have dropped to zero at thefirst resonant frequency of the actuator, which in FIG. 4 is shown asbeing approximately 2,400 Hz. The selected structure therefore allowsthe cutting speed to be set to 50 meters per minute since the spatialfrequency characteristic of the actuator has dropped to approximately2,350 kHz which is near zero. The desired surface generated by means ofthe random-check generator, as shown in FIG. 6, which corresponds to thespatial structure shown in FIG. 5, therefore has the amplitude responsecharacteristic of white noise, limited by low-pass filtering, and canadvantageously be produced with the aid of the actuator 30 according tothe invention with a cutting speed of approximately 50 m/min. Thisallows working of even very hard metal materials, such as hard alloys,that require sufficiently high cutting speed, as otherwise vibrationwould occur.

FIG. 8 illustrates the general control of the device 10 used for forminga microstructure, when working in the way of a facing operation.

As a first step, a stochastic lens microstructure is produced (compareFIGS. 5 and 6, respectively). Alternatively, it is of course alsopossible to image other structures, such as converted photos, symbols,etc.

These structures are then transformed to polar coordinates.

Then follow the selection of the working conditions and the simulationof the working conditions, based on a spatial frequency analysis. Usinga suitable cutting speed, a real-time computer used for driving theactuator, which may consist of a PC with signal processor card, thengenerates the necessary actuation signals for the actuator and forpositioning the actuator in radial direction. The real-time computerevaluates for this purpose the pulses generated by the encoder 31 forthe axis of rotation and the pulses generated by a further encoder forthe radial axis, and generates the initiating and the terminating pulsesfor the working operation.

The spatial frequency analysis determines the cut-off frequency of thedesired microstructure to be produced. If the spatial frequency analysisindicates a very high cut-off frequency, this has the result that a verylow cutting speed can be selected only to ensure that the maximumcut-off frequency will remain below the first resonant frequency of theactuator. This may be the case, for example, if the desiredmicrostructure to be produced comprises sharp edges or the like.

It would then be convenient to first smooth the desired microstructureby application of a low-pass filter or a band-pass filter, for example aSobel filter, in order to obtain a low-pass filtered signal that can beworked with advantage using the actuator system according to theinvention.

The desired microstructure to be produced (compare FIG. 6) should have asquare shape, with an odd number of imaged pixels of, for example, 801by 801 pixels. Based on that square structure, it is then possibleduring the facing operation to work a circle of 801 pixels in diameter.In order to obtain square pixels also at the outer contour of the mage,the image format should correspond approximately to the resolution ofthe encoder for the spindle axis, divided by PI (pixel accuracy). If thedepth is selected to be 1 Byte (=255 grey levels), the encoder used mayhave 2,500 increments, for example. By way of example, the 255 greylevels should later correspond to a depth of 0 to 10 μm for the depthfeed motion of the actuator.

FIG. 9 now shows the relevant algorithm for such a working operation.

The desired microstructure to be produced is converted to polarcoordinates and is stored as a polar-coordinate image G(c, n) in a LUTwith 2,500×401 pixels, the actuation value of the actuator being givenwith an accuracy of 1 Byte. The actuation value G may thus assume valuesof between 0 and 255, which may correspond to an actuator excursion ofbetween 0 and maximally 40 μm, or else to a smaller range of, forexample, 0 to 10 μm.

The polar-coordinate image illustrated in FIG. 9 is stored in the LUT inthe BMP format.

As illustrated in FIG. 9, the angular position of the spindle 12 ispicked up by means of the spindle C encoder and is recorded via the edgecounter, and processed as a “c value”, based on an initial “spindleinitiation” value, after conversion to a digital value.

Likewise, the radial position of the actuator is picked up by an encoderY, and is processed via an edge counter after conversion to a digitalvalue.

The piezo drive is controlled via a real-time computer using theactuation values for the z direction (travel of h_(min) to h_(max)). Theamplifier receives its input values from the LUT and from the valuesreceived from the encoder for the angular position and the radialposition, via a digital-to-analog converter, according to a suitablealgorithm.

The amplifier input voltage U_(min) at g=0 corresponds to h_(min), whilethe amplifier input voltage U_(max) at g=255 corresponds to the maximumactuation value h_(max). The term “complete radius” describes the numberof radial pulses for the travel from R_(max) to R₀ (i.e. from the outerradius to the center). The term “floor(y)” in FIG. 9 indicates that thenumber has been rounded to the next lower even number.

Using the algorithm illustrated in FIG. 9, the amplifier is controlledvia the digital-to-analog converter, based on the values from the LUTand the encoder C values for the spindle and the radial encoder Y forthe radial position.

The algorithm makes use of interpolation for two immediate adjacentpixels in the radial direction to smooth the amplifier input voltage byone pixel in radial direction for the feed motion of the actuator. As isshown in the first box at the right beside the polar-coordinate image,the algorithm interpolates g=G(c, n+1)*(1−d)+G(c, n+2)*d. From this, thedigital value u for the amplifier input voltage is obtained asu=U_(min)+g*(U_(max)−U_(min))/255. This value is converted by thedigital-to-analog converter to an analog value for controlling theamplifier.

Thus, linear interpolation of the actuating values is carried out foradjacent pixels in the axial direction, while no interpolation isperformed for the angle of rotation. One thereby avoids “turning marks”showing in the turning direction, which otherwise would be noticeable.

No interpolation is required for the actuation value at constant radialposition, the mechanical system of the actuator having sufficiently highinertia to provide a smoothing effect in the circumferential direction.

In FIG. 10 a longitudinal section through an actuator according to theinvention is shown which is slightly modified with respect to theembodiment shown in FIG. 2 and which is designated with numeral 30′ intotal. Herein corresponding parts are designated with correspondingreference numerals.

The actuator 30′ largely corresponds in its design to the actuatorpreviously described with respect to FIG. 2. By contrast to theembodiment of FIG. 2 now the tool holder 42 is positioned so that thetool is precisely center-aligned. Thereby force components perpendicularto the axial direction, which would result in an off-axis configuration,can be avoided. Thus the cutting-power noise is further reduced.

In addition, the centering between the ram 30′ and the piezo drive 36 isdesigned with a crowned surface 88 at the screw 80 and with a flatcounter surface 90 at the piezo drive 36.

Finally, in addition between the sintered plate 66 and the membrane 68or the counter membrane 69, respectively, a small gap 73 of a thicknessof about 0.1 to 1 millimeters, preferably of 0.1 to 0.5 millimeters, isformed which is filled with a damping medium. This can for instance beair, grease or oil.

Also thereby the cutting-power noise is further reduced.

1. A device for producing microstructures, comprising: a spindle which can be rotated about a longitudinal axis thereof; a fixture provided on said spindle for clamping a workpiece; a linear drive adapted to produce a linear feed motion in a first direction; an actuator arranged on said linear drive; wherein said actuator comprises: a fast drive for driving a tool in a direction substantially perpendicular to a workpiece surface; a guide coupled with said fast drive and said tool; wherein said fast drive is configured for generating a fast advancing motion of said tool toward a workpiece surface against the action of a restoring force; and wherein said guide is configured to allow feeding of the tool in an axial direction of said fast drive, and to resist movement of said tool in a plane perpendicular to that direction.
 2. The device of claim 1, wherein said fast drive is configured as a piezo drive.
 3. The device of claim 1, wherein said guide has a static rigidity of at least 100 N/μm, in a plane perpendicular to an axial direction of said guide.
 4. The device of claim 1, wherein said guide comprises a ram movably supported within a housing against a resilient force.
 5. The device of claim 4, wherein said ram is supported by first and second spring elements, said spring elements being substantially unyielding in a radial direction of said ram, while allowing deflection in an axial direction of said ram against a spring force of said spring elements.
 6. The device of claim 5, wherein said spring elements are configured as leaf springs that have lengthwise ends clamped in holders and that can be moved substantially transversely to a plane wherein said leaf springs extend.
 7. The device of claim 6, wherein said spring elements are configured as feeler gauge strips that are clamped in radial direction in crossed arrangement between said holders and said ram.
 8. The device of claim 5, wherein said spring elements are configured as disk springs.
 9. The device of claim 4, wherein said ram is biased against said fast drive by a fluid pressure.
 10. The device of claim 8, wherein said fast drive further comprises a pressure-tight space formed within said housing and being sealed toward the outside, in axial direction, by a first diaphragm connected with said ram, and by a second diaphragm connected with said ram on a side facing said tool; wherein a fluid pressure can be applied to said pressure-tight space; and wherein said first diaphragm has a larger surface exposed to said fluid pressure than has said second diaphragm.
 11. The device of claim 4, wherein a first end of said fast drive opposite said ram is clamped on a holding fixture, and wherein a second end of said fast drive, opposite the first end, is coupled with said ram via a compensating element configured for compensating alignment errors between a longitudinal axis of said ram and a longitudinal axis of said fast drive.
 12. The device of claim 4, wherein said housing comprises an attenuator for damping axial movement of said fast drive.
 13. The device of claim 10, further comprising an attenuator for damping axial movement of said fast drive; wherein said attenuator comprises a sintered material comprising an open porosity; and wherein a gap being filled with a damping medium is formed between said attenuator and one of said diaphragms.
 14. The device of claim 1, further comprising a controller for controlling a movement of said actuator, said controller being configured for controlling movement of said actuator relative to a workpiece in response to an angular position of said workpiece and a position of said actuator along a feed direction of said linear drive.
 15. The device of claim 14, wherein said controller comprises means for transforming a specified microstructure to be produced on the workpiece into a coordinate-transformed structure defined by polar coordinates, which contains actuation values for said actuator as a function of an angle of rotation of said workpiece and a linear feed of said linear drive in a direction along a workpiece surface.
 16. The device of claim 15, wherein said controller comprises means for transforming a specified microstructure to be produced on the workpiece, defined by Cartesian coordinates, to a coordinate-transformed structure defined by polar coordinates, where said actuation values for said actuator are stored as a function of polar coordinates which contain an angle of rotation of said workpiece and a radial distance of said fast tool from an origin of the polar coordinate system.
 17. The device of claim 15, wherein the coordinate-transformed structure is stored in a look-up table LUT, from which said electronic controller derives an actuation signal that is supplied to an amplifier for driving the actuator.
 18. The device of claim 17, wherein said controller comprises means for interpolating the actuation signal supplied to the amplifier as a function of position increments of the linear feed motion of said linear drive.
 19. A device for producing microstructures, comprising a spindle which can be rotated about a longitudinal axis thereof; a fixture provided on said spindle for clamping a workpiece; an linear drive adapted to produce a linear feed motion in a first direction; an actuator arranged on said linear drive; wherein said actuator comprises a fast drive for driving a tool in a direction substantially perpendicular to a workpiece surface; a guide coupled with said fast drive and said tool; wherein said fast drive is configured for generating a fast advancing motion of said tool toward a workpiece surface against the action of a restoring force; wherein said guide is configured to allow feeding of the tool in an axial direction of said fast drive, and to resist movement of said tool in a plane perpendicular to that direction; a controller for controlling a movement of said actuator, said controller being configured for controlling movement of said actuator relative to a workpiece in response to an angular position of said workpiece and a position of said actuator along a feed direction of said linear drive; wherein said actuator has a first resonant frequency of at least 1500 Hz; and wherein said controller is configured for generating a filtered signal for driving said actuator, said filtered signal being selected from the group formed by a low-pass filtered and a band-pass filtered signal, said filtered signal having an upper cut-off frequency being lower than a resonant frequency of said actuator.
 20. An actuator for moving a workpiece on a turning machine for the purpose of producing a microstructured surface by means of a fast drive; wherein said fast drive is coupled with a tool via a ram coupled to a guide allowing feeding the tool in an axial direction of the fast drive against the action of a restoring force, and resisting a movement of the tool in a plane perpendicular to that direction; and wherein said ram is biased against said fast tool by a fluid pressure. 